though no two are created equal. Turbo upgrades in itself offer a wide range of possible results, plus factor in fuel types as you mentioned and the variables becomes almost endless. I think it would be best to generalize more regarding power enhancing modifications. It can easily become a slippery slope leading to overall goal focus becoming lost. As I mentioned before, organizing it based on some form of "normalcy" tends to define mostly what goes into each category or stage. Anyone deviating from that course does that more out of "wanting to be different", researching component limitations, or "while I'm there I should do this to prepare for that down the line" (timeline/cost efficiency based).

Owners will and have installed upgraded turbos on a single intake, stock intake piping, or stock exhaust manifolds, etc. However, I would still consider them a "Stage 5-ish" Z, and a weak one at that. You could simplify it further and divide it strictly into: Bolt-ons, Tuned or Un-Tuned, Small or Large Turbos, Built Shortblock or Longblock. Done.

So returning to your observations & suggestions, there is quite a load of variability going on, but going into too much detail is straying from the original intention of the chart, right? Fuel types produce drastically different results, as do different turbos - that's fine. Variability within the stages themselves is OK, varying outside of stages is considered unusual or incomplete, simply because we know what the potential of a given sum of parts should be. We expect people to install certain things to support other things. If we were all supposed to arrive at the same HP, we wouldn't have much of a choice in parts. Correct, we could technically make a chassis/suspension modification chart that is separate from the power mod chart, to quickly show the level of completion for that. It probably wouldn't be difficult either unlike the power related chart.

So I guess the question is, is it worth making a modern version? If so, based on HP (Stage 0 = Stock, Stage 1 = XXX HP over stock)? Individual or groups of modifications installed? Old charts were almost listed as each item equals a stage, yet since technology has come so far, we would have 40 stage Z's or more. The criteria can be changed to many different facets to reflect a given range of goals and potential.